Marketing, SEO, and growth baseline

- Added contact page with honeypot and optional Resend delivery.
- Terms and privacy static pages, changelog with RSS, docs help center.
- Newsletter capture API and footer signup (single opt-in; double opt-in documented as follow-up).
- Env-gated analytics script hooks, sitemap, robots, and Open Graph defaults.
- Marketing chrome respects `product_locale` cookie (EN/ES) alongside `/settings/locale`.